TERRESTRIAL EVOLUTION OF CLAY MINERALS

The clay mineralogy of the Earth has changed significantly over its ~4.56 Byr history, due to changes in water availability and composition, rock type, atmospheric composition and climate, and biota. Clay minerals were largely absent prior to planetessimal formation but are abundant today in all near-surface Earth environments. The evolution of clay minerals as products of chemical and biological weathering of surface and near-surface rocks is an important part of Earth’s mineralogical history, and five major modes of clay mineral paragenesis began at different stages of Earth’s history: (1) subsurface aqueous/hydrothermal alteration largely to serpentine and chlorite minerals; (2) authigenesis, especially from solution in marine sediments; (3) low-grade metamorphism (to greenschist facies) and subsequent exposure through orogenesis related to plate tectonics; (4) near-surface weathering, especially under oxic and/or acidic conditions, most important after the great Oxidation Event; and (5) the rise of a terrestrial biosphere, most notably the advent of soil-forming microbes, fungi, and plants and associated biological weathering. Particularly important events affecting clay mineral formation include development of the continents, onset of plate tectonics, an increase in atmospheric oxygen, and the development of life. The rise in atmospheric oxygen not only ultimately gave rise to widespread life but also greatly affected weathering and aqueous chemistry (e.g., Fe). Clay mineral formation has the potential to affect Earth’s geosphere and biosphere in many ways. For example serpentinization of olivine can produce large amounts of H₂. Clay minerals have diverse and flexible structures, with the potential to host much of the periodic table in their tetrahedral, octahedral, and interlayer sites. Their structures can also include H₂O molecules, species such as NH₄⁺, and they strongly adsorb many organic molecules. Indeed, they have been implicated in sequestration of organic carbon, linked to an increase in atmospheric oxygen. Clay minerals may have played critical roles in the origin of life, and life has subsequently played dramatic roles in the formation of near-surface clay minerals. Clay minerals are thus excellent examples of the co-evolution of Earth’s geosphere and biosphere.